Motor control system including motor protector and remote sensor for controlling motor operation

ABSTRACT

The control system employs a thermostatic motor protector for controlling a system parameter, such as temperature, at a distance from the motor as well as for protecting the motor. A thermistor is provided having a resistance which varies in response to changes in the value of the parameter. The thermistor is interconnected with a heater associated with the motor protector to cause the protector to deenergize the motor if the value of the parameter exceeds a predetermined threshold.

United States Patent Buiting 1 Jan. 18, 1972 154] MOTOR CONTROL SYSTEMINCLUDING MOTOR PROTECTOR AND REMOTE SENSOR'FOR CONTROLLING MOTOROPERATION [72] Inventor: Francis P. Buiting, Plainville, Mass.

[73] Assignee: Texas Instruments Incorporated, Dallas,

Tex.

[22] Filed: Aug. 25, 1970 [21] Appl. No.: 66,897

Related US. Application Data [63] Continuation of Ser. No. 729,123, May14, 1968,

abandoned.

52 use: 511 lnt.Cl. H02h5/04 5s Eie1dofSeareh..-...3l8/22lR,221l-1,227,471,

[56] References Cited UNlTED STATES PATENTS 2,874,344 2/1959 Slocum..318/473 2,930,959 3/1960 Slocum ..3 18/221. 1 3,079,524 2/1963 Gibson..3 18/473 3,255,397 6/1966 Vaughan... .....3l8/221 3,290,576 12/1966Jensen ..318/471 Primary Examiner-T. E. Lynch Assistant Examiner-W. E.Duncanson, Jr.

Attorney-Harold Levine, Edward J. Connors, Jr., James P. McAndrews, JohnA. l-laug and Gerald B. Epstein [57] ABSTRACT motor if the value of theparameter exceeds a predetermined threshold.

I 15 Claims, 5 Drawing Figures aisasnzs PATENTED JAN 8 I972 FIGQI llll.

FIG.4

FIGS

May I 4, 1968 now abandoned.

This invention relates to a motor control and more particularly to sucha control for controlling the energization of an a electric motor theoperation of which affects a variable parameter at a distance from themotor.

In typical controls in which a variable parameter is controlled orlimited by a control circuit, separate contacts or switching means areprovided for motor control and motor protection. For example, the motoritself may be protected by a thermostatic switch embedded in thewindings of the motor while the control function is provided by apower-switching relay apart from the motor; Thus, two power-switchingmechanisms must be provided.

Among the several objects of the present invention may be noted theprovision of a control for an electric motor which employs a singleswitch means or set of contact for both motor protection and forcontrolling a variable system parameter; the provision of such a systemin which an essentially conventional thermostatic motor protectorcontrols the operation of an electric motor to prevent a systemparameter from going beyond a preselected value; the provision of such asystem which employs solid-state sensing means; and the provision ofsuch a system which is reliable and which is relatively simple andinexpensive. Other objects and features will be in part apparent and inpart pointed out hereinafter.

Briefly, a control according to the present invention is operative tocontrol the energization of an electric motor the operation of whichaffects the value of at least one variable parameter at a distance fromthe motor. The control includes a thermostatic motor protectorassociated with the motor. The protector includes atemperature-sensitive switch for interrupting the fiow of current froman electric power source to the motor to deenergize the motor uponoverloading thereof and the protector also includes a heater which is inheat exchange relationship with the switch for operating the switch. Athermistoris provided having a resistance which varies in response tochanges in the value of the parameter. The thermistor and the heater areinterconnected for varying the energization of the heater as a functionof the value of the parameter. Accordingly, the motor is deenergized ifthe value of the parameter crosses a predetermined threshold.

The invention accordingly comprises the constructions hereinafterdescribed, the scope of the invention being indicated in the followingclaims.

In the accompanying drawings in which various possible embodiments ofthe invention are illustrated,

FIG. 1 is a schematic circuit diagram of a control of this invention inwhich line voltage is applied across a serially connected thermistorsensor and a thermostatic switch operating heater;

FIG. 2 is a schematic circuit diagram of another embodiment of theinvention wherein a thermistor sensor is connected in shunt with aswitch-operating heater;

FIG. 2A illustrates a modification of the embodiment of FIG. 2 employinga plurality of thermistor sensors;

FIG. 3 is a schematic circuit diagram of a further embodiment wherein asensor and its heater are energized from the voltage drop providedacross a heater which is in series with the motor windings; and

FIG. 4 is a schematic circuit diagram of an embodiment of this inventionfor use with a three-phase motor.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

Referring now to FIG. I, there is indicated at P1 an essentiallyconventional thermostatic motor protector Pl. Protector P1 is associatedwith a motor M which it is designed to protect and may, for example, bemounted within the motor housing or embedded within the windings of themotor itself so as to be responsive to heat generated therein. Theprotector includes a thermostatic switch SW which may, asdiagrammatifunction by self-heating.

cally illustrated, comprise a snap-acting, bimetallic disc whichcooperates with fixed contacts to break a circuit to the motor when thetemperature of the disc exceeds a predetermined level. The protectoralso includes a first heater H1 which is connected in series with themotor M and the switch SW across a pair of AC supply leads L1 and L2 andwhich is in heat exchange relationship with the disc. As is understoodby those skilled in the art, the heat generated by heater HI varies asthe function of the current drawn by the motor M so that if the motor isstalled or is energized in a locked rotor condition so that overloadcurrents are drawn, the switch SW is quickly heated to its actuationtemperature to open the motor circuit thereby deenergizing andprotecting the motor M. As is understood by those skilled in the art,the disc itself may be constructed so as to possess sufficientresistance to provide this The protector Pl also includes an auxiliaryheater H2 which is in heat exchange relationship with thetemperature-sensitive switch SW. Auxiliary heater H2 is connected acrosssupply leads L1 and L2 through a circuit which includes the thermostaticswitch SW and a sensing thermistor THl. Sensing thermistor THl islocated at a distance from the motor and responds to a variable systemparameter the value of which is affected by the operation of the motor.As an example of a system in which the present control is useful, themotor M may comprise the blower motor for a conventional hot airfurnace. In such systems it is desirable to deenergize the furnaceincluding the motor M if the temperature in the furnace plenum exceeds apredetermined level or threshold. Such overtemperature operation might,forv example, be caused by the failure of a drive belt interconnectingthe motor M and the blower so that plenum circulation is interruptedeven though the motor M itself continues to run normally. The thermistorTl-Il is then mounted in the plenum to sense the temperature there andis constructed of a semiconductor material having an NTC (negativetemperature coefficient of resistivity) characteristic.

Such a control system then operates substantially as follows. Assumingthat the plenum temperature is below the predetermined threshold andthat the motor M is operating normally, the heaters H1 and H2 do notgenerate sufficient heat to bring the switch SW to its operatingtemperature. Thus the energization of the motor M is maintained. If themotor is overloaded for any reason, the heater H1 generates sufficientheat to raise the temperature of the switch SW so as to actuate it todeenergize the motor in conventional fashion. If the plenum temperatureshould rise above the predetermined threshold temperature, theresistance of the thermistor TI-Il decreases thereby increasing theenergization of the heater H2. This increased energization of the heaterH2 raises the temperature of the switch SW to a level above itsoperating point, deenergizing the motor M and shutting off the furnace.

To provide a sharply defined plenum temperature at which deenergizationoccurs, it is preferable that the temperature-resistance characteristicof the semiconductor material comprising thermistor THl have a sharpknee, that is, the material should itself have a threshold temperatureabove which the resistance of the material decreases relativelyabruptly. In this way, a switching-type action is provided which gives awell defined point of deenergization of the motor M.

The resetting of the protector may be either automatic, occurring uponcooling down of the thermostatic bimetal disc, or manual, as isunderstood by those skilled in the motor protector 811.

A similar mode of operation may be provided using a PTC (positivetemperature coefi'icient of resistivity) thermistor as illustrated inFIG. 2. In this embodiment, a protector P2 is employed which comprisesthermostatic switch SW and a single heater H3 in heat exchangerelationship with the switch, the auxiliary heater being omitted. Theheater H3 is connected in series with the motor M across supply leads L1and L2. However, in this modification, the heater H3 is shunted by a PTCthermistor TH2 which is responsive to plenum temperature.

The characteristics of the heater H3 are chosen so that the normalrunning current drawn by the motor M will cause that heater to generatesufficient heat to raise the temperature of the thermostatic switch SWto its operating point, provided the heater H3 carries the entire motorcurrent. However, as may be seen from the circuit diagram of FIG. 2, aportion of the motor current is carried by the PTC thermistor TH2. Aslong as thermistor TH2 is relatively cool, the shunting action providedby the thermistor prevents the tripping of switch SW by the heatgenerated in heater H3. if the plenum temperature should rise above thepredetermined threshold, the resistance of thermistor TH2 increases sothat the heater H3 carries substantially the entire motor current andthus the switch SW is operated to deenergize the motor. Thus,substantially the same mode of operation is obtained as in theembodiment illustrated in FIG. 1. Again it is preferable that thesensing thermistor have switching-type characteristics. In other words,it is preferable that the PTC thermistor TH2 be constructed of asemiconductor material having a transition temperature above which theresistance of the material rises relatively abruptly. Such materials arewell known in the art and are conventionally employed forself-regulating heaters.

if desired, the single PTC thermistor TH2 can be replaced with aplurality of similar PTC thermistors TH2A, TH2B and THZC connected inseries as illustrated in FIG. 2A. As is apparent to those skilled in theart,'an increase in resistance in any one of the three thermistors willhave substantially the same effect on the operation of the control as anincrease in the resistance of the single thermistor in the embodiment ofF IG. 2.

The circuit illustrated in FIG. 3 is useful when it is not desirable toapply the full supply voltage across the ,sensing thermistor and theheater associated therewith, as was done in the example of FIG. 1. Inthe HO. 3 circuit, a remote sensing thermistor TH3 of the NTC type isconnected in series with an auxiliary protector heater H and theseries-connected pair is connected across a main heater H4. The mainheater H4 is connected in series with the motor M. The sensing circuitis thus energized by the voltage drop developed across heater H4 by thecurrent drawn by the motor M. lt can be seen that the energization ofthe heater H5 varies not only as a function of the resistance of thesensing thermistor TH3 but also as a function of the motor current sincethe motor current determines the voltage supplied to the sensingcircuit. In many applications, this interaction is desirable in that itis advantageous to have the sensitivity of the sensing circuit increaseas the load on the motor M increases.

The present invention may also be applied to a system employing athree-phase motor MY as shown in FIG. 4. The motor MY comprises threewindings WA, WB and WC which are arranged in a wye configuration, oneend of each winding being connected to a respective supply lead LA, LBand LC. An essentially conventional three-phase motor protector P4 isassociated with motor MY. Protector P4 includes three sets of contactsKA, KB and KC and a bimetallic operator K for opening these contactswhen the temperature of the operator exceeds a predetermined level.Heaters HA, HB and HC are respectively connected in series with themotor windings and 4 the respective contacts. When the contacts KA-KCare closed they complete the wye connection of the motor windings andestablish a neutral terminal or lead as indicated at N. Protector P4further comprises a heater H6 which is connected in series with a remotesensing thermistor TH6 between one of the supply leads (LC) and theneutral lead N.

As is understood by those skilled in the art, the protector P4 willprotectively deenergize the motor MY if anyone of the three windingsWA-WC draws an overload current through the respective heater HA-HC.Further, if the temperature of the thermistor TH6 passes a predeterminedthreshold, which may be either a relatively high temperature or arelatively low temperature depending upon whether the thermistor TH6 isof the PTC or NTC type', the heater H6 is energized and causes theoperator K to open the contacts KAKC thereby deenergizing the motor.

While a hot air furnace has been described as a principal example forthe purpose of illustration, it should be understood that otherparameters in other systems may be similarly sensed to initiate motorenergization or deenergization when the parameter crosses apredetermined threshold. For example, the compressor motor in arefrigeration system can be deenergized if the condenser temperatureexceeds a given threshold or if the evaporator temperature falls below apredetermined threshold. lt is also understood by those skilled in theart that thermistors operated in a self-heating mode may be employed asflow rate detectors. Thus the sensing thermistor in any one of theembodiments illustrated can be used in a furnace to sense whether adraft is present for the burner or it can be used in a blower system tocause whether there is an obstruction blocking flow. Further, thesensing thermistor can be employed to provide thermostatic control on asustained basis for an area to which heat or cooling is being providedby the operation of the controlled motor. Many other applications willbe readily apparent to those skilled in various other arts.

Similarly, it will be understood by those skilled in the art that aplurality of thermistors may be used in any one of the embodimentsillustrated in place of the one thermistor shown if it is desired tosense a plurality of different parameters or if it is desired to sensethe same parameter in a plurality of locations. The thermistors may thenbe interconnected in logical fashion to provide a change in theoperation of the motor only upon desired combinations of conditions. Forexample, various logical gating functions using thermistors suitable forthis purpose are illustrated in copending application, Ser. No. 668,509,filed Sept. 18, 1967. Similarly, the operation of the motor can beprogrammed according to a desired sequence by controlling the heating ofthe sensing thermistor with electrothermal timing or sequencingapparatus of the type disclosed in copending application, Ser. No.608,230, filed Jan. 9, 1967. Further, the heaters used herein maythemselves be PTC or NTC thermistors to give even greater flexibility inswitching as taught by the copending applications, supra.

ln view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

l. A control for an electric motor the operation of which motor affectsthe value of at least one variable parameter of a system at a distanceremote from said motor, said control comprising:

a thermostatic motor protector including a temperaturesensitive switchfor selectively interrupting the flow of current from an electric powersource to said motor, said protector including a heater in heat exchangerelationship with said switch for effecting operation thereof, saidheater being adapted to effect operation of said temperature-sensitiveswitch in response to overloading of said motor and in response topredetermined changes in the variable parameter in the remote system,

a thermistor having a resistance which varies in response to changes inthe value of said parameter, said thermistor being disposed at alocation remote from said motor, and in thermal communication with thevariable parameter; and

means interconnecting said heater and said thermistor for varying theenergization of said heater as a function of the value of the variableparameter to effect deenergization of the motor when the value of theparameter crosses a predetermined threshold.

2. A control as set forth in claim 1 wherein said thermistor and saidheater are connected in series across said electric power source.

3. A control as set forth in claim 2 wherein said thermistor has anegative temperature coefficient of resistivity.

4. A control as set forth in claim 3 wherein said thermistor has atransition temperature above which the resistance of said thermistordecreases relatively abruptly.

'5. A control as set forth in claim 1 wherein said protector furtherincludes a second heater, said second heater being in heat exchangerelationship with said switch and connected in series with said motorfor operating said switch when said motor draws an overload current.

6. A control as set forth in claim 5 wherein said first heater and saidthermistor are connected in series across said second heater whereby thesensitivity of said thermistor to variations in the value of saidparameter varies as a function of the current drawn by said motor.

7. A control as set forth in claim 1 wherein said heater is connected inseries with said motor for operating said switch to deenergize saidmotor when said motor draws an overload current.

8. A control as set forth in claim 7 wherein said thermistor isconnected across said heater.

9. A control as set forth in claim 8 wherein said thermistor has apositive temperature coefficient of resistivity.

10. A control as set forth in claim 9 wherein said thermistor has atransition temperature above which the resistance of said thermistorincreases relatively abruptly.

11. A control as set forth in claim 1 wherein said switch comprises asnap-acting bimetal disc.

12. A control for an electric motor the operation of which motor affectsthe value of at least one variable parameter of a system at a distanceremote from said motor, said control comprising:

. a thermistor having a resistance which varies in response to changesin the value of said parameter, said thermistor being disposed at alocation remote from said motor; and

a thermostatic motor protector including a single temperature-sensitiveswitch for selectively interrupting the flow of current from an electricpower source to said motor in response to overloading of the motor andin response to changes in the resistance of said remote thermistor, saidprotector including a first heater in heat exchange relationship withsaid switch and connected in series with said motor for operating saidswitch when said motor draws an overload current and a second heaterelectrically coupled to said remote thermistor the energization of saidsecond heater being controlled by the resistance of said remotethermistor, said second heater also being in heat exchange relationshipwith said switch for effecting operation of said switch in response topredetermined variations in the parameter to effect deenergization ofthe motor when the value of said parameter crosses a predeterminedthreshold.

13. A control as set forth in claim 12 wherein said thermistor and saidsecond heater are connected in series across said source.

14. A control for a three-phase electric motor having three windings,the operation of which motor affects the value of at least one variableparameter of a system at a distance from said motor, said controlcomprising:

a thermistor having a resistance which varies in response to changes inthe value of said parameter; and

a thermostatic motor protector including three heaters each respectivelyconnected in series with one of said windings and temperature-sensitiveswitching means in heat exchange relationship with said heaters forinterrupting the flow of current from an electric power source to saidwindings, said protector including also a fourth heater the energizationof which is controlled by the resistance of said thermistor said fourthheater also being in heat exchange relationship with said switch wherebysaid motor is deenergized if the value of said parameter crosses apredetermined threshold.

15. A control as set forth in claim 14 wherein said switching meansconnects said windings to a neutral terminal and wherein said thermistorand said fourth heater are connected in series across one of saidwindings.

1. A control for an electric motor the operation of which motor affectsthe value of at least one variable parameter of a system at a distanceremote from said motor, said control comprising: a thermostatic motorprotector including a temperature-sensitive switch for selectivelyinterrupting the flow of current from an electric power source to saidmotor, said protector including a heater in heat exchange relationshipwith said switch for effecting operation thereof, said heater beingadapted to effect operation of said temperature-sensitive switch inresponse to overloading of said motor and in response to predeterminedchanges in the variable parameter in the remote system, a thermistorhaving a resistance which varies in response to changes in the value ofsaid parameter, said thermistor being disposed at a location remote fromsaid motor, and in thermal communication with the variable parameter;and means interconnecting said heater and said thermistor for varyingthe energization of said heater as a function of the value of thevariable parameter to effect deenergization of the motor when the valueof the parameter crosses a predetermined threshold.
 2. A control as setforth in claim 1 wherein said thermistor and said heater are connectedin series across said electric power source.
 3. A control as set forthin claim 2 wherein said thermistor has a negative temperaturecoefficient of resistivity.
 4. A control as set forth in claim 3 whereinsaid thermistor has a transition temperature above which the resistanceof said thermistor decreases relatively abruptly.
 5. A control as setforth in claim 1 wherein said protector further includes a secondheater, said second heater being in heat exchange relationship with saidswitch and connected in series with said motor for operating said switchwhen said motor draws an overload current.
 6. A control as set forth inclaim 5 wherein said first heater and said thermistor are connected inseries across said second heater whereby the sensitivity of saidthermistor to variations in the value of said parameter varies as afunction of the current drawn by said motor.
 7. A control as set forthin claim 1 wherein said heater is connected in series with said motorfor operating said switch to deenergize said motor when said motor drawsan overload current.
 8. A control as set forth in claim 7 wherein saidthermistor is connected across said heater.
 9. A control as set forth inclaim 8 wherein said thermistor has a positive temperature coefficientof resistivity.
 10. A control as set forth in claim 9 wherein saidthermistor has a transition temperature above which the resistance ofsaid thermistor increases relatively abruptly.
 11. A control as setforth in claim 1 wherein said switch comprises a snap-acting bimetaldisc.
 12. A control for an electric motor the operation of which motoraffects the value of at least one variable parameter of a system at adistance remote from said motor, said control comprising: a thermistorhaving a resistance which varies in response to changes in the value ofsaid parameter, said thermistor beinG disposed at a location remote fromsaid motor; and a thermostatic motor protector including a singletemperature-sensitive switch for selectively interrupting the flow ofcurrent from an electric power source to said motor in response tooverloading of the motor and in response to changes in the resistance ofsaid remote thermistor, said protector including a first heater in heatexchange relationship with said switch and connected in series with saidmotor for operating said switch when said motor draws an overloadcurrent and a second heater electrically coupled to said remotethermistor the energization of said second heater being controlled bythe resistance of said remote thermistor, said second heater also beingin heat exchange relationship with said switch for effecting operationof said switch in response to predetermined variations in the parameterto effect deenergization of the motor when the value of said parametercrosses a predetermined threshold.
 13. A control as set forth in claim12 wherein said thermistor and said second heater are connected inseries across said source.
 14. A control for a three-phase electricmotor having three windings, the operation of which motor affects thevalue of at least one variable parameter of a system at a distance fromsaid motor, said control comprising: a thermistor having a resistancewhich varies in response to changes in the value of said parameter; anda thermostatic motor protector including three heaters each respectivelyconnected in series with one of said windings and temperature-sensitiveswitching means in heat exchange relationship with said heaters forinterrupting the flow of current from an electric power source to saidwindings, said protector including also a fourth heater the energizationof which is controlled by the resistance of said thermistor said fourthheater also being in heat exchange relationship with said switch wherebysaid motor is deenergized if the value of said parameter crosses apredetermined threshold.
 15. A control as set forth in claim 14 whereinsaid switching means connects said windings to a neutral terminal andwherein said thermistor and said fourth heater are connected in seriesacross one of said windings.